Degradation Behavior of Nanoreinforced Epoxy Systems Under Pulsed Laser by Dr Aglan

8/12/2019 Degradation Behavior of Nanoreinforced Epoxy Systems Under Pulsed Laser by Dr Aglan

1/9

Degradation Behavior of Nanoreinforced Epoxy SystemsUnder Pulse Laser

M. Calhoun,1 A. Kumar,2 H. Aglan1

1Mechanical Engineering Department, Tuskegee University, Tuskegee, Alabama 360882Physics Department, Tuskegee University, Tuskegee, Alabama 26088

Received 9 September 2008; accepted 17 November 2008DOI 10.1002/app.29887Published online 7 May 2009 in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: Nanocomposites using EPON 824 as theirmatrix were exposed to pulse laser at 532 nm for varioustime intervals. The developed nanomaterials used for thisstudy were manufactured using EPON 824 with multi-walled carbon nanotubes (MWCNTs) at a loading rate of0.15% by weight and nanoclays at a loading rate of 2% byweight as reinforcements. The effect of laser irradiation on

polymer composites has been investigated. The degrada-tion mechanism for the epoxy was of a laser inducedburning nature. Of all specimens tested, the ultimatestrength of the MWCNT-reinforced specimens decreasedthe most as a function of radiation time; the nanoclay-rein-forced epoxy retained the most strength after 2 min of

laser radiation. In addition, the threshold fluence fordecomposition indicated that less energy was required toinitiate decomposition in the MWCNT-reinforced epoxythan in the nanoclay-reinforced epoxy. This can be attrib-uted to the high thermal conductivity of the carbon nano-tubes. Measurement of surface damage in the material wasobserved via electron microscopy. Fourier transform infra-

red spectroscopy was used to investigate changes to themolecular structure as a function of exposure time. VVC 2009Wiley Periodicals, Inc. J Appl Polym Sci 113: 31563164, 2009

Key words: nanocomposites; mechanical properties;morphology; degradation; FTIR

INTRODUCTION

Durability of polymeric systems and their compo-sites under various environmental conditions is a

major concern for scientists and engineers, in thatthis ultimately determines the suitability of a mate-rial for specific applications. Understanding thedegradation mechanisms for these systems undervarious challenging environmental conditions isextremely important in the determination of fatiguelife. These environmental challenges include ultra-violet radiation, thermal events, moisture, corrosion,erosion among others.1 Although there have beenmany studies reported in the literature concerningpolymer degradation,29 there has not been as muchconsideration toward investigating the degradationmechanisms of nanocomposites.

Polymers have been recognized for their use inimproving material properties in specific applica-tions, solar energy storage technology, and photo-sensitized reduction of water in microheterogeneous

systems, photoelectrical systems, artificial photosyn-thesis, and spacecrafts.1017 It is well known that theperformance of polymers degrades with continuousexposure to electromagnetic and solar radiation. Theimportance of this phenomenon is underscored incommercial and governmental sectors. Developmentof materials that are stable under extreme environ-mental conditions can significantly mitigate the hugecosts associated with bridge and marine craft repairdue to corrosion, road and structural repair due torepeated heat/cool cycles, as well as photodegrada-tion of coatings on boats, aircrafts, and militaryvehicles. Ongoing development and testing of mate-rials is critical toward this effort.

Carbon nanotubes and nanoclays have receivedconsiderable mention in recent years, each becauseof its own set of superior properties. Multiwalled

carbon nanotubes (MWCNTs) can improve upon thethermal and mechanical properties of a polymermatrix. It has been well documented that MWCNTshave superior properties such as Youngs modulus(1 TPa), tensile strength (20100 GPa), thermal con-ductivity [6000 W/(m K)], high aspect ratio, andspecific strength (48.5 MN m/kg), among others.18

Ganguli et al.19 studied the effect of loading rate andsurface modification of MWCNTs on fracture tough-ness using a bifunctional epoxy as the polymermatrix. It was found that the addition of 0.15%MWCNTs, by weight, had an 80% improvement on

Journalof AppliedPolymer Science,Vol. 113,31563164(2009)VVC 2009 Wiley Periodicals, Inc.

Correspondence to: H. Aglan ([email protected]).Contract grant sponsor: US Department of Energy;

contract grant number: DE-FG52-05NA27039.Contract grant sponsor: National Science Foundation

MRI Grant; contract grant number: CMS-0619827.


2/9

fracture toughness for that system. Likewise, it hasbeen demonstrated that nanoclays in thermoset poly-mers can improve fracture toughness.20,21 Smectictype clays, including montmorillonite, are attractivefor use as fillers because of their lightweight nature.

Their superior wetting abilities make them verypromising in the field of barrier protection againstcorrosion.22

The degradation mechanisms of polymers whichhave been exposed to ultraviolet radiation (UV)have been examined. Aglan et al. examined theeffect of UV radiation on polyurethane elastomers. Itwas found that discoloration appeared after only 1month of exposure and that the tearing energy ofthe material decreased by 60% after 3 months and98% after 5 months. DSC indicated a cleavage of theurethane bonds that was indicated by an increasingendothermic reaction with time.23 In a companionstudy, Aglan and coworkers24 confirmed the break-age of the urethane bonds facilitating aggregation

between hard and soft segments in the molecularchains. Further, the increase in the melting enthalpyindicated an increase in this phase separation as afunction of exposure time. Woo et al.25,26 studied theeffects of UV radiation on organoclayepoxy compo-sites and found that the nanocomposite had shal-lower microcracks than the neat species after 300 hof exposure. In Additional, increasing the percentageof nanoclay lowered the tensile strength and strainto failure while increasing the modulus. This trendwas unchanged as a function of UV radiation expo-sure, indicating that nanoclay loading rate, not UVradiation exposure, is the dominant factor in deter-mining tensile strength. Even with accelerated UVtest chambers, penetration of UV radiation, particu-larly through opaque materials, can be time consum-ing. Lasers can be used to study the degradationmechanism of these materials on a rapid time scale.

Laser radiation can interact with polymers via avariety of processes, depending on the frequencyand power of the laser radiation and the energy

band gap of the exposed material. If the polymerhas an energy band which matches with the energyof an incoming photon, the light energy is absorbed.Generally, polymers will release absorbed energy as

a nonradiative process, which leads to an increase inthe temperature of the interaction zone, and finally,thermal damage in the neighborhood of the exposedarea occurs. If the energy of the light is less than theenergy band of the polymer, the light can beabsorbed via a process called multiphoton absorp-tion, followed by nonradiative relaxation andheating. However, the probability of multiphotonabsorption is much less than of linear (single-photon) absorption. Another process called radiativeablation is almost independent of frequency ofincoming photons, and it is caused by a strong elec-

tric field of photons sufficient to ionize atoms in theplasma state. This can be achieved by focusing thelaser light at a spot on the top of the material. Theaverage etch depth per pulse on a polymer surfaceexhibits a logarithmic dependence on the incident

laser fluence (E) above a fluence threshold value(Eth) and is given by:

Lf1

emln

E

Eth; (1)

where e is extinction coefficient (also called molarabsorptivity in l mol/cm). The symbol m indicatesmolar concentration (mol/L). E is intensity of laser

beam, andEth is intensity threshold for onset of pho-todecomposition. The value of Eth can be obtainedexperimentally by plotting a graph between Lf andln(E). Its intercept can give the value of ln Eth, andslope of the graph gives the value of 1/em.

27

Possible mechanisms for etching vary betweentype of polymer and frequency of incident radiation.Perhaps the most likely mechanisms are the absorp-tions which promote thermal excitation in a sample,which means that the mechanism of etching frompulse to pulse can be different. The very first laserpulse may not create any etching in the sample, butit warms up and facilitates the burning and etchingfrom the second pulse and onwards. In the presentstudy, we used an unfocused laser beam from a fre-quency doubled YAG (neodymium-doped yttriumaluminum garnet; Nd:Y3Al5O12) laser (532 nm),which is used to study the degradation of epoxy

and epoxy nanocomposites by surrounding electro-magnetic radiation on a rapid time scale. NeatEPON 824 and two nanoreinforcements, namely anorganophilic layered silicate and MWCNTs, wereexposed to the laser for 30 s, 1 or 2 min. The me-chanical properties, fracture surface morphology,and changes in absorbencies as measured by Fouriertransform infrared spectroscopy are reported.

MATERIALS

EPON 824

The structured epoxy resin used in this study was

EPON 824, which was provided by Hexion SpecialtyChemicals, Houston, TX. The epoxy resin is a highpurity epichlorohydrin epoxy bisphenol A. The sec-ond part of the system is Epi-Cure 3277, which is apolyamide adduct inn-butanol.

Nanoclay

The nanoclay (NC) used for this study is a naturalmontmorillonite, which was provided by SouthernClay, Gonzales, TX. The nanoclay was modified withan ammonium salt to make it organophilic.

DEGRADATION BEHAVIOR OF NANOREINFORCED EPOXY SYSTEMS 3157

Journal of Applied Polymer Science DOI 10.1002/app


3/9

Multiwalled carbon nanotubes

MWCNTs were also used as nanoreinforcement forthe epoxy system. The MWCNTs used in this study

were provided by Ahwahnee Technologies, San Jose,CA. The diameter of these tubes was in the range of215 nm, with a length of 110 lm, and 520 layers.

EXPERIMENTAL

Specimens were prepared in neat, 2% nanoclay, and0.15% MWCNT formulations. The mix ratio used forresin to hardener was 2 : 1. All specimens werecured in an oven at 30C for 16 h.

Nanoclay/EPON 824 nanocomposites

Nanoclay was added to the epoxy resin in smallincrements (1015% of the total weight to be added)followed by mixing in a high shear mixer to removeagglomerates. Once all nanoclay was added to theresin, the mixture was left overnight to allow thor-ough wetting of the nanoparticles. After the hard-ener was added to the resin in the proper ratio, highshear mixing at 1800 rpm for three cycles of 30 swas used to ensure a well-dispersed system. Themixture was then poured into dogbone molds usinga target of 3 mm thickness, and it was cured as

previously described.

MWCNT/EPON 824 nanocomposites

The EPON 824 samples were also prepared in 0.15%MWCNT formulations by weight. A solution withan epoxy resin to MWCNTs ratio of 50 : 1 was pre-pared by high shear mixing. A 2% MWCNT/epoxyresin solution was prepared beforehand and dilutedto achieve the desired 0.15% concentration. Sampleswere prepared as described above.

YAG pulse laser

This study employed a YAG (neodymium-dopedyttrium aluminum garnet; Nd:Y3Al5O12) laser. A

schematic of the Nd: YAG laser setup is given inFigure 1. A dichroic mirror that reflects the 532 nmat 45 from the direction of the incident beam andtransmits the residual first harmonic from theNd:YAG laser was used to guide the 532 nm radia-tion (pulse duration 5 ns) onto the sample. Thesamples were exposed for the time durations of 30 s,1 min, and 2 min. The average laser power used forexposing the samples was 2.6 W.

Tensile testing

The tensile strength of specimens was evaluated

using a Sintec 5D Material Testing System. All speci-mens were tested at a crosshead speed of 12.7 105 m/s. The samples were dogbone shaped, witha gauge length of 88 mm, target thickness of 3 mm,and width of 12.7 mm at the most narrow point.

Microscopy

Fracture surfaces and crater morphology of epoxysamples were examined using a Hitachi S-2150 scan-ning electron microscope (SEM). Samples wereobserved at various magnifications with an accelerat-ing voltage of 10 kV. Prior to being put into thevacuum chamber, samples were mounted with con-ductive tape and sputter coated with Au-Pd alloy.Optical micrographs were captured with a WildHeerbrugg M3Z microscope.

Fourier transform infrared spectroscopy

Fourier transform infrared spectroscopy (FTIR) spec-tra were captured using a Nicolet 6700 FTIR spec-trometer with an attenuated total reflectance (ATR)accessory. The FTIR spectrometer has a spectralresolution of 4 cm1 over a 4004000 cm1 wave-number range.

Figure 1 Schematic of Nd:YAG pulse laser setup. [Color figure can be viewed in the online issue, which is available atwww.interscience.wiley.com.]

3158 CALHOUN, KUMAR, AND AGLAN



4/9

RESULTS AND DISCUSSION

Mechanical properties

All tensile tests were repeated a minimum of threetimes for each formulation and all exposure times.Representative specimens are reported. Figure 2shows the stressstrain relationship of the unagedsamples for the neat, 0.15% MWCNT/epoxy, and2% nanoclay (NC)/epoxy nanocomposites. As can

be seen in Figure 2, the ultimate strengths of theunaged specimens are approximately 59, 64, and 57

MPa for the neat epoxy, 0.15% MWCNT/epoxy, and2% NC/epoxy nanocomposites, respectively. Theaddition of the MWCNT to the epoxy resin yieldeda slight improvement in the ultimate strength overthe neat epoxy, whereas the addition of the nanoclayresulted in a slight decrease in the ultimate strengthin the epoxy nanocomposite. It should be noted,however, that this decrease is nominal and is notstatistically significant (Table I). The strain to failurein both nanocomposite systems improved over theneat epoxy, and the 2% NC/epoxy formulationexhibits some plastic deformation, indicating anincrease in ductility. It is seen from Figure 2, based

on the first linear portion of the stressstrain behav-ior of the three materials, that there is no significant

change in stiffness with such loadings of thenanoparticles.

Figure 3 shows neat and nanoreinforced epoxy af-ter 2 min of laser radiation. The ultimate strength ofthe neat epoxy, 0.15% MWCNT/epoxy, and 2% NC/

epoxy nanocomposites is 22, 17, and 40 MPa, respec-tively. The neat and 0.15% MWCNT/epoxy sampleshad significant loss in both ultimate strength andstrain to failure after 2 min laser exposure as com-pared to the unaged specimens. The 2% NC/epoxyretained most of its ultimate strength with increasingexposure times, retaining almost 70% of its originalultimate strength after 2 min of laser radiation ascompared to the unaged specimens. The neat epoxyretained about 40% of its original ultimate strengthafter 2 min exposure, whereas the 0.15% MWCNT/epoxy lost almost 75% of its original strength.

Table I shows the ultimate strengths for the threeformulations at various time intervals of laser expo-sure. It can be seen that after 30 s, the ultimatestrengths for the specimens were 52, 46, and 50 MPafor the neat epoxy, 0.15% MWCNT/epoxy, and 2%NC/epoxy nanocomposites, respectively. The neatepoxy and the 2% NC/epoxy nanocomposites bothhad a decrease in ultimate strength of about 7 MPaover their unaged formulations after 30 s of laserradiation. However, the 0.15% MWCNT/epoxynanocomposite specimen had a decline of 18 MPa,representing a 28% decrease in ultimate strengthover the unexposed specimens. It is clear that afteronly a short exposure to the laser, the 0.15%MWCNT/epoxy specimen lost a significant amountof its tensile strength. After 1 min, there was nosignificant difference in the ultimate strength of the2% NC/epoxy specimen, whereas the ultimatestrength of the neat epoxy and the 0.15% MWCNT/epoxy decreased by 23 and 50% over their unagedspecies, respectively. It is evident from this data thatthe nanoclay reinforcement enabled the epoxy to

better resist loss in mechanical strength due to laserexposure.

Relationship between energy fluenceand crater depth

After the nanoreinforced specimens were exposed tolaser radiation for a period of time, craters were

TABLE IUltimate Strengths (MPa) of Neat and Nanostructured Epoxy Unexposed and After

Various Pulse Laser Exposure Times

Formulation

Exposure

Unaged 30 s 1 min 2 min

Neat epoxy 57 5.1 54 3.2 44.2 1.5 23.7 5.42% NC/epoxy 56.1 1.5 51 4.0 54.2 0.8 39.4 6.00.15% MWCNT/epoxy 62.5 2.9 48 1.0 31.4 1 15.9 1.1

Figure 2 Stressstrain relationship of unaged neat andnano-reinforced epoxy. [Color figure can be viewed in theonline issue, which is available at www.interscience.wiley.com.]




5/9

visible in them. It is believed that the nanoparticlescreated thermally active sites, thus creating craters atthe laser exposed sites. The laser fluence in thisstudy is above the threshold for ablation; hence, thecrater formation is due to thermal decomposition(pyrolysis) and burning of the nanostructured epoxy

by successive absorption of the laser pulse energy.Equation (1) was modified to fit our experimentalconditions in order to determine the threshold flu-ence for thermal decomposition. The accumulativetotal energy was calculated as the product of totaltime of exposure and laser power. The maximum

area of the exposed top surface was measured to be6.35 105 m, based on the area of the laser. Foreach specimen type, the average etch depths wereplotted against lnE, the energy fluence per unit areaat 30, 60, and 120 s. The plot was fitted to a straightline, from which the fluence threshold was deter-mined from the y-intercept of the plot. The resultsare shown in Table II. It was determined thatthe thresholds of accumulative laser fluence fordecomposition in the 2% NC/epoxy and the 0.15%MWCNT/epoxy nanocomposites were 154.5 and112.5 J/cm2, respectively. This indicates that moreaccumulated energy was needed to begin decompo-

sition in the 2% NC/epoxy nanocomposite than in

the 0.15% MWCNT/epoxy nanocomposite. The dif-ference in the fluence energies between the 2% NC/epoxy and the 0.15% MWCNT/epoxy can be attrib-uted to the higher thermal conductivity of theMWCNT composite. This is based on our measure-ments of thermal conductivity of 5% by weight load-ing for both the MWCNT/epoxy and NC/epoxynanocomposites, which revealed that the MWCNT/epoxy nanocomposite has a thermal conductivity of0.27 W/(m K), whereas the NC/epoxy has a thermalconductivity of 0.197 W/(m K). The neat epoxy was

also measured, and it was found to have a thermalconductivity of 0.194 W/(m K). The high thermaltransport in carbon nanotubes is facilitated by thecrystalline lattice structure of the nanotubes, com-prised only of carbon atoms,28 which offers multipletransport paths as compared to the amorphousstructure of the epoxy matrix or the plate-like struc-ture of the nanoclay. Thus the MWCNT/epoxy com-posite displays a higher capability for transportingheat than the nanosilicate composites.

The depth of the craters formed in the materialsas a result of laser radiation is shown in Table III.As can be seen in the table, crater formation in the

0.15% MWCNT/epoxy specimens occurred fasterand was more pronounced than either the 2% NC/epoxy or the neat specimens. This data is in agree-ment with the fluence threshold data which suggeststhat less accumulated energy was required to beginthermal decomposition in the 0.15% MWCNT/epoxythan in the 2% NC/epoxy. As previously mentioned,MWCNTs have a robust thermal transport mecha-nism, which is evidenced by an average crater depthafter 1 min of laser radiation of 0.48 0.07 (15.24%penetration). The nanoclay loading rate used in thisstudy is more than 10 times that of the MWCNTloading by weight, yet there was no sign of crater

formation as a result of laser radiation in the 2%

TABLE IIThreshold Fluence for Decomposition of

Nanostructured Epoxy

2% NC/epoxy 0.15% MWCNT/epoxy

Threshold

fluence (J/cm2

) 154.5 112.5

TABLE IIIDepth of Craters from Thermal Decomposition as a Result of Laser Exposure in Neat and Nanostructured Epoxy

Neat epoxy 2% NC/epoxy 0.15% MWCNT/epoxy

30 s 1 min 2 min 30 s 1 min 2 min 30 s 1 min 2 min

Ave spl thickness (mm) 0.00 0.00 0.00 3.66 0.17 3.37 0.3 3.42 0.21 3.25 0.15 3.17 0.10 3.35 0.24Ave depth penetration

(mm) 0.00 0.00 0.00 0.00 0.00 0.95 0.05 0.11 0.02 0.48 0.07 1.13 0.17Penetration through

sample (%) 0.00 0.00 0.00 0.00 0.00 27.99 2.85 3.47 0.4 15.24 2.04 34.00 7.22

Figure 3 Stressstrain relationship of neat and nano-rein-forced epoxy exposed to Nd:YAG Laser for 2 min. [Colorfigure can be viewed in the online issue, which is availableat www.interscience.wiley.com.]




6/9

NC/epoxy specimens after 2 min of exposure. Thisis due to the insulative properties of the nanoclayand the epoxy. For the neat specimens, however, itwas apparent visually that the laser caused some in-ternal damage. As the neat epoxy is transparent, theenergy deposited on the sample from the laser per-meated through the top layers and began to propa-gate, creating cracks and clefts in the interior of thespecimens, which were seen at the fracture surface[Fig. 4(a)].

Composite fractographs for the neat epoxy, the 2%NC/epoxy, and the 0.15% MWCNT/epoxy are

shown in Figure 4(ac). As can be seen in Figure4(a), there is severe internal damage to the specimenas a result of the laser exposure. The neat epoxy dis-sipated the laser energy throughout the material,creating an area characterized by cracks and rough-ness on the fracture surface. However, as the laserdamaged the specimen prior to fracture, the featuresseen in the fractograph are indicative of the laserdamage and not of resistance to fracture. Figure4(b,c) show the fracture surface of the 2% NC/epoxyand the 0.15% MWCNT/epoxy after 2 min of laser

radiation. There are craters present at the exposedareas because of laser radiation. The crater forma-tions indicate that the laser damage was localized inthe nanophase specimens. A rough area is observed

just after the indentation in the 2% NC/epoxy speci-men. This area just after the crater formation is in-dicative of the initial energy required to begin thefracture. This feature was not present in the 0.15%MWCNT/epoxy specimen and indicates that theMWCNT-reinforced specimen was less resistant tofracture than the NC-reinforced specimen. Thiscorrelates with the mechanical testing; the ultimate

strength of the MWCNT-reinforced materialdecreased the most of all specimens tested as a func-tion of duration of laser radiation.

Surface morphology

Optical and scanning electron micrographs showingthe surface morphology at the laser exposed sitesare seen in Figures 5 and 6, respectively. The micro-graphs in Figure 5(ac) detail the laser spot at 6.5magnification for the neat epoxy, 2% NC/epoxy,

Figure 4 Composite fractographs of epoxy specimens exposed to Nd:YAG laser for 2 minutes at 500 magnification:(a) neat EPON 824, (b) 2% NC/epoxy and (c) 0.15% MWCNT/epoxy.

Figure 5 Optical microscope images of the laser damaged area of the (a) neat epoxy, (b) 2% NC/epoxy and (c) 0.15%MWCNT/epoxy. [Color figure can be viewed in the online issue, which is available at www.interscience.wiley.com.]




7/9

and 0.15% MWCNT/epoxy, respectively. As can beseen in Figure 5(a), there appears to be cracking justoutside of the laser exposed area in the neat epoxy.This is consistent with the cracking seen in the frac-ture surface. There does not appear to be any char-ring on the laser exposed surface of the neat sample,whereas the 2% NC/epoxy and 0.15% MWCNT/ep-oxy [Fig. 5(b and c), respectively] do show evidenceof charring. This is also consistent with the fracturesurfaces, as etching occurred in the nanoreinforcedspecimens, forming craters. The area of the localizeddamage to the 0.15% MWCNT/epoxy samples ismeasurably larger than that of the 2% NC/epoxysamples.

Scanning electron micrographs at 300 are givenin Figure 6(ac), and they show the morphology ofthe laser exposed area of the neat epoxy, 2% NC/epoxy, and 0.15% MWCNT/epoxy, respectively. Asexpected, cracks are visible on the surface of thelaser exposed area of the neat epoxy [Fig. 6(a)].These cracks occurred as the neat epoxy absorbedenergy given off by the laser. Given that epoxy is acrosslinked system, there was limited molecular mo-

bility as the energy from the laser propagatedthrough the specimens. In addition, there was nomelting of the thermoset epoxy; hence, cracksformed in the material in order to dissipate theenergy from the laser. Figure 6(b,c) shows the mor-phology of the area inside the craters formed by the

laser for the 2% NC/epoxy and 0.15% MWCNT/ep-oxy, respectively. Pitting can be seen on the surfacein both specimens. These pits range in size from 5to 20 lm for the 2% NC/epoxy and 210 lm forthe 0.15% MWCNT/epoxy. It is believed that thesepits are the result of pyrolysis. It has been reportedthat MWCNT deposited on a thin silicon filmexposed to Nd:YAG at 532 nm, can produce surfaceheat of as much as 1503C in 13 ns.29 The tempera-tures produced under sustained laser pulses are suf-ficient for pyrolysis to occur. The surface featuresseen in the nano reinforced specimens are likely to

be due to gasses escaping from the matrix as it issoftened by the laser radiation and the nanoparticles

boring through the epoxy matrix.

FTIR analysis

FTIR spectra for the neat, 0.15% MWCNT/epoxy,and 2% NC/epoxy nanocomposites are shown inFigure 7(ac). The spectra for all formulations showabsorption bands at approximately the same wave-numbers for no exposure and 30 s and 2 min of laserexposure. It can also be seen in the figures that forthe nanostructured formulations, the relative inten-sities of the major peaks decay as a function of expo-sure time to laser radiation. The amount of peakdecay seen correlates with the threshold energiesrequired for degradation of the specimens. Similar

behavior has been previously reported for thermo-setting polymer systems under CO2 laser radiation.

30

Figure 7(b,c) shows representative spectra for the0.15% MWCNT/epoxy and the 2% NC/epoxy atdifferent exposure times. Peak decay correlating toCH3 symmetrical bending in the bisphenol A struc-ture is observed at 1378 and 1369 cm1. The primaryaliphatic amine of the curing agent is observed at1103 and 1089 cm1. The peak seen at 930 cm1 cor-responds to stretching of the CAO bonds. The

degree to which we see peak decay is directlyrelated to the threshold for decomposition in thepolymers. Recall that the 2% NC/epoxy specimenshad no measurable decomposition craters after 30 sof laser radiation, whereas the 0.15% MWCNT/ep-oxy had already begun to degrade after 30 s of expo-sure. This trend is also present in the FTIR spectra.The neat epoxy spectra show no signs of peak decay.There was a small peak that developed at 1548cm1. It is believed this peak developed because ofdegradation products; however, further investigationis required to designate this peak.

Figure 6 EM micrographs of 2 min laser exposed area at 300: (a) neat epoxy, (b) 2% NC/epoxy and (c) 0.15%MWCNT/epoxy.




8/9

CONCLUSIONS

Neat EPON 824 epoxy and nanocompositesmanufactured with MWCNTs and nanoclayswere subjected to Nd:YAG laser radiation fordifferent time intervals to study their degrada-

tion behavior. Prior to laser exposure, the ulti-mate strengths of the neat epoxy, 0.15%MWCNT/epoxy, and 2% NC/epoxy were 59,64, and 57 MPa, respectively. After 2 min of ex-posure, the ultimate strengths of the neat epoxy,

0.15% MWCNT/epoxy, and 2% NC/epoxy haddecreased to 23.7, 15.9, and 39.4 MPa,respectively.

After exposure of only 30 s, both the neat andthe 2% NC/epoxy showed no apparent damage,whereas the 0.15% MWCNT/epoxy had cratersdue to decomposition on its surface. The fracturesurface of the three materials after 2 minshowed that the neat epoxy had a crevice in theinterior of the sample but no crater on the laserexposed side. The 2% NC/epoxy and the 0.15%MWCNT/epoxy specimens both had beenetched from laser radiation; however, the inden-tations on the MWCNT-reinforced samples weremuch deeper and the fracture surface was muchsmoother than in the 2% NC/epoxy, indicatingthat the material was less resistant to fracture asa function of exposure time.

The threshold fluence for laser decompositionwas lower for the 0.15% MWCNT/epoxy thanthe 2% NC/epoxy; hence, less energy wasrequired to decompose the MWCNT-reinforcedmaterial. This is attributed to the high thermalconductivity of the MWCNT, which acceleratedpyrolysis in the nanocomposite.

FTIR showed peak decay with respect to expo-sure time in the nanoreinforced polymers. Therate of peak decay correlates to the threshold flu-ence of decomposition. There was no peak decayobserved in the neat epoxy.

References

1. Pandey, J. K.; Reddy, K. R.; Kumar, A. P.; Singh, R. P. PolymDegrad Stab 2005, 88, 234.

2. Pandey, J. K.; Singh, R. P. e-Polymers 2004, 51, 1.3. Huaili, Q.; Chungui, Z.; Shimin, Z.; Guangming, C.; Mingshu,

Y. Polym Degrad Stab 2003, 81, 497.

4. Morlat, S.; Mailhot, B.; Gonzalez, D.; Gardett, J. Chem Mater2004, 16, 377.5. Mailhot, B.; Morlat, S.; Gardett, J.; Boucard, S.; Duchet, J.;

Gerard, J. Polym Degrad Stab 2003, 82, 163.6. Sloan, J. M.; Patterson, P.; Hsieh, A. Polym Mater Sci Eng

2003, 88, 354.7. Pramoda, K. P.; Liu, T.; Liu, Z.; He, C.; Sue, H.-J. Polym

Degrad Stab 2003, 81, 47.8. Du, J. X.; Wang, D. Y.; Wilkie, C. A.; Wang, J. Q. Polym

Degrad Stab 2003, 79, 319.9. Bourbigot, S.; Gilman, J. W.; Wilkie, C. A. Polym Degrad Stab

2004, 84, 483.10. Klosterman, D.; Chartoff, R.; Graves, G.; Osborne, N.; Light-

man, A.; Han, G.; Bezeredi, A.; Rodrigues, S. Ceram Eng SciProc 1998, 19, 291.

Figure 7 FTIR for unexposed, 30 s and 2 min laser expo-sure times: (a) neat epoxy, (b) 0.15% MWCNT/epoxy and(c) 2% NC/epoxy. [Color figure can be viewed in theonline issue, which is available at www.interscience.wiley.com.]




9/9

11. Lerch, B. A.; Draper, S. L.; Baaklini, G. Y.; Pereira, L. M. InHiTemp Rev 1999: Advanced High Temperature Engine Mate-rials Technology Project; 1999; Vol. 2, p 30.

12. Gyekenyesi, A. L.; Baaklini, G. Y. In Proceedings from SPIE;Baaklini, G. Y.; Nove, C. A.; Boltz, E. S., Eds.; Bellingham,WA, 2000; Vol. 3585, p 142.

13. Gyekenyesi, A. L.; Baaklini, G. Y. In Proceedings from SPIE;Baaklini, G. Y.; Nove, C. A.; Boltz, E. S., Eds.; Bellingham,WA, 2000; Vol. 3993, p 78.

14. Abdul-Aziz, A.; Baaklini, G. Y.; Zagidulin, D.; Richard, W.;Rauser, R. W. In Proceedings from SPIE; Baaklini, G. Y.; Nove,C. A.; Boltz, E. S., Eds.; Bellingham, WA, 2000; Vol. 3993, p 35.

15. Li, J.; Tong, L.; Fang, Z.; Gu, A.; Xu, Z. Polym Degrad Stab2006, 91, 2046.

16. Kashiwagi, T.; Grulke, E.; Hilding, J. Macromol Rapid Com-mun 2002, 23, 761.

17. Lie, D. L.; Mullan, C.; Favre, S.; OConnor, G. M.; Glynn, T. J.Proceedings of the Third International WLT: Conference onLasers in Manufacturing, Munich, Germany, June 2005.

18. Available at: http://www.Ahwahneetech.com. Accessed onJune 9, 2008.

19. Ganguli, S.; Aglan, H.; Dennig, P.; Irvin, G. J Reinforced Plast

Compos 2006, 25, 175.

20. Kim, B. C.; Park, S. W.; Lee, D. G. Comp Struct 2008, 86, 69.21. Wang, L.; Wang, K.; Chen, L.; Zhang, Y.; He, C. Compos A

2006, 37, 1890.22. Allie, L.; Thorn, J.; Aglan, H. J Appl Polym Sci 2008, 50, 2189.23. Aglan, H.; Calhoun, M.; Allie, L. J Appl Polym Sci 2008, 108,

558.24. Ludwick, A.; Aglan, H.; Abdalla, M. O.; Calhoun, M. J Appl

Polym Sci 2008, 110, 712.25. Woo, R. S. C.; Chen, Y.; Zhu, H.; Li, J.; Kim, J.; Leung, C. K. Y.

Compos Sci Technol 2007, 67, 3448.26. Woo, R. S. C.; Chen, Y.; Zhu, H.; Li, J.; Kim, J.; Leung, C. K. Y.

Compos Sci Technol 2008, 68, 2149.27. Rabek, J. F. In Photodegradation of Polymers: Physical Charac-

teristics and Applications; Springer-Verlag: Heidelberg, 1996;Chapter 8.

28. Gojny, F. H.; Wichmann, M. H. G.; Fiedler, B.; Kinloch, I. A.;Bauhofer, W.; Windle, A. H.; Schulte, K. Polymer 2006, 47,2036.

29. Nakamiya, T.; Ueda, T.; Ikegami, T.; Mitsugi, F.; Ebihara, K.;Tsuda, R. Diamond Relat Mater, to appear.

30. Bormashenko, E.; Pogreb, R.; Sheshnev, A.; Shulzinger, E.;Bormashenko, Y.; Sutovski, S.; Pogreb, Z.; Katzir, A. Polym

Degrad Stab 2001, 72, 125.


Journal of Applied Polymer Science DOI 10 1002/app

Documents

Degradation Behavior of Nanoreinforced Epoxy Systems Under Pulsed Laser by Dr Aglan